In the early stage, a scanning apparatus is used to scan the image of a single document. After the document has been scanned, the document should be removed from the scanning apparatus and then a next document could be placed on the scanning apparatus in order to be further scanned. Since the process of manually replacing the document is very troublesome, the conventional scanning apparatus is not feasible to scan a stack of documents. Recently, an automatic document feeder is usually integrated into the scanning apparatus. After a stack of documents to be scanned are placed on the sheet input tray of the automatic document feeder, the automatic document feeder will successively transport the documents to perform a scanning operation without the need of manually replacing the documents. This means of automatically feeding the documents is both time-saving and efficient.
Hereinafter, the configurations and operating principles of a conventional automatic document feeder will be illustrated with reference to FIG. 1. FIG. 1 is a schematic view illustrating a conventional automatic document feeder. The conventional automatic document feeder 1 comprising a casing 10, a sheet input tray 11, a pick-up module 12, a transfer channel 13, a sheet ejecting tray 14, plural transfer roller assemblies 15 and an ejecting roller assembly 16. A scanning module 17 with a scanning function is disposed under the automatic document feeder 1. The document D to be scanned is placed on the sheet input tray 11. The pick-up module 12 is disposed in the vicinity of the sheet input tray 11 for feeding the document D into the transfer channel 13. The transfer roller assemblies 15 are arranged in the transfer channel 13 for transporting the document D through the transfer channel 13. After the document D is scanned, the scanned document D is ejected to the sheet ejecting tray 14 and supported on the sheet ejecting tray 14.
After the scanning operation is started, the document D placed on the sheet input tray 11 is fed into the transfer channel 13 by the pick-up module 12. After the document D is fed into the transfer channel 13, the document D is transported to a scan region above the scanning module 17 by the transfer roller assemblies 15, which is arranged in the transfer channel 13. Once the document D is transported across the scan region above the scanning module 17, the image of the document D is read out by the scanning module 17. The, the scanned document D is transported to the sheet ejecting tray 14 by the transfer roller assemblies 15 and the ejecting roller assembly 16. Meanwhile, the scanning operation of the document D is completed.
The conventional automatic document feeder 1, however, still has some drawbacks. For example, when a stack of documents D are fed by the automatic document feeder 1, a so-called double feeding or multi feeding problem occurs. Once the double feeding or multi feeding problem occurs, two or more documents are simultaneously fed into the transfer channel 13 to be scanned. Since two or more documents are partially or completely superimposed with each other when the double feeding or multi feeding problem occurs, missing pages are possibly generated. Moreover, if many documents are simultaneously fed into the transfer channel, the components within the automatic document feeder 1 are possibly abraded. In this situation, the use life thereof is shortened.
Therefore, there is a need of providing an improved automatic document feeder for avoiding the double feeding or multi feeding problem.